Field of the Invention
The present invention relates to a linear transmission device, and more particularly to an offset preload ball screw with expandable loading area.
Description of the Prior Art
Ball screw is a reliable, quiet and high precision linear transmission device, and is normally applied with a preload in order to ensure high precision of positioning.
As shown in FIG. 1, a conventional offset preload ball screw 10 comprises a screw 11 and a nut 12 mounted on the screw 11. The screw 11 includes a screw helical groove 111, the nut 12 includes a nut helical groove 121, and a plurality of balls 13 is disposed between the screw and nut helical grooves 111, 121. As shown in FIG. 3, which shows the relationship between the axial coordinates and the helical angle coordinates of the nut helical groove 121, wherein the preload is applied in such a manner that an offset point P is provided on the nut helical groove 121, the horizontal axis represents the helical angle coordinates of the nut helical groove 121, and the longitudinal axis represents the axial coordinates of the nut helical groove 121. At the left side of the offset point P, the helical angle coordinates of the nut helical groove 121 is directly proportional to the axial coordinates of the nut helical groove 121, the amount of increase in the axial coordinates for each additional 360 degrees (namely one revolution of the nut 12) of the helical angle coordinates is defined as a lead value L. At the right side of the offset point P, the helical angle coordinates of the nut helical groove 121 is also directly proportional to the axial coordinates of the nut helical groove 121, namely, the lead value L. However, at the offset point P, the nut helical groove 121 produces a lead offset value δ, which will cause axial coordinates of the nut helical groove 121 offset, and therefore the balls 13 at two sides of the offset point P will produce preloads in opposite directions. The lower part of FIG. 1 shows the preload change, wherein the horizontal axis represents the axial coordinates, and the longitudinal axis represents the preload applied to the balls 13 in the axial coordinate direction. The negative and positive signs indicate the direction of the preload, and the lower part of FIG. 1 clearly shows that the direction of the preload applied to the balls 13 of the ball screw 10 changes sharply at the offset point P. At the left side of the offset point P, the value of the preload applied to the ball screw 10 is positive, and the value of the preload of the ball screw 10 at the right side of the offset point P becomes negative all of a sudden. The slope of the preload is infinite, and the differential of the curve at this point can also be defined as infinite. The sharp change in preload would cause instable and unsmooth running of the ball screw 10. FIG. 2 shows another ball screw 10. The preload change of the balls 13 is shown in the lower part of FIG. 2, where the slope of the preload at the offset point P is also infinite, therefore the ball screw 10 as shown in FIG. 2 suffers the same disadvantages of the ball screw as shown in FIG. 1.
As shown in FIGS. 4 and 5, another ball screw 20 also comprises a screw 21, a nut 22, and a plurality of balls 23 disposed in a circulation path between the screw 21 and the nut 22. The circulation path includes a loading section A, a non-loading section B, and a loading section C. The loading sections A and C at both sides of the non-loading section B are subjected to preloads in opposite directions, and the arrangement of the non-loading section B releases the preloads applied to the area between the loading sections A and C. By such arrangements, the problem that the preload applied to the ball screw changes sharply from positive value to negative value at such a rate that the slope is infinite can be prevented. However, the lower part of FIG. 5 shows the preload change, wherein the horizontal axis represents the axial coordinates, and the longitudinal axis represents the preload applied to the balls 23 in the axial coordinate direction. The negative and positive signs indicate the direction of the preload, and the lower part of FIG. 5 clearly shows that the direction of the preload applied to the balls 23 of the ball screw 20 still changes sharply at the two end point of the non-loading section B, namely, the preload applied to the balls in the non-loading section B will be reduced sharply to 0. Therefore, the ball screw 20 will still encounter sharp preload change, which consequently will cause noise and vibration when the ball screw 20 is running. Furthermore, the balls 23 in the non-loading section B cannot be subjected to the loading between the screw 21 and the nut 22, and therefore does not offer any help to the load capacity of the ball screw 10.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.